Plastic semiconductor packages are fabricated using lead frames. A conventional plastic package includes a semiconductor die attached to a lead frame, and encapsulated in a plastic body. The lead frame supports the die during a molding process, and provides the internal traces and terminal leads of the completed package. Typically, the lead frame comprises a stamped or etched metal, such as a nickel-iron alloy, or a copper based alloy. Lead frames are provided in strips adapted to form multiple packages, which are subsequently singulated into individual packages.
One type of lead frame includes planar mounting paddles for the dice, and patterns of lead fingers spaced around the peripheries of the mounting paddles. Prior to the molding process, the backsides of the dice can be attached to the mounting paddles using an adhesive layer. In addition, metal wires can be wire bonded to the lead fingers, and to bond pads on the faces of the dice. Using a transfer molding process, a plastic material, such as an epoxy resin, is molded to either side of the dice and lead frame, and over the wire bonded wires to form package bodies. The lead frame is then singulated into multiple packages, and the lead fingers are trimmed and shaped to form the terminal leads of the packages.
Each semiconductor package includes a semiconductor die attached to a mounting paddle and wire bonded to multiple lead fingers, and a plastic body encapsulating the die and portions of the lead fingers. Sometimes defects can occur in this type of package as a result of bowing of the mounting paddle. For example, bowing can result from thermal stresses developing between the metal mounting paddle and the plastic body of the package. The thermal stresses occur because a coefficient of thermal expansion (CTE) for the mounting paddle is different than a CTE of the plastic body. Bowing of the mounting paddle resulting from the thermal stresses, can cause the plastic body to crack, or to delaminate, from the lead frame.
Defects can also occur in this type of package as a result of defects in the adhesive layer formed between the die and the mounting paddle. For example, the adhesive layer can have a varying thickness, and discontinuous portions where there is no adhesive material. These defects can cause the die to pop loose from the mounting paddle, or can cause cracking in the package body. One method for controlling the thickness of the adhesive layer is by forming bumps on the mounting paddle, and then pressing the die into contact with the bumps. This method is disclosed in U.S. Pat. No. 5,214,307, to Davis.
Yet another source of defects in plastic packages results from an orientation and location of the mounting paddles with respect to other components of the lead frame. Typically, the mounting paddles include support members on either end, which attach to spaced side rails of the lead frame. These support members are sometimes referred to as "tie bars". The support members are relatively thin segments of metal formed continuously with the mounting paddles and side rails. Twisting and bending of the support members during stacking and handling of the lead frame, and during the semiconductor packaging process, can change the orientation and location of a mounting paddle, and of a die attached to the mounting paddle. This can cause defects in a subsequently formed plastic body, and in the bond lines between the plastic body and the lead frame.
In view of these and other deficiencies in conventional lead frames, improvements in lead frames, and in fabrication methods for semiconductor packages, are needed in the art.